This invention relates to improved water based coating compositions for use as a primer-surfacer over metal prior to application of a top coat of an acrylic type enamel or lacquer. It is directed specifically to water based compositions for this purpose in which the resinous film-forming vehicle and binder for any pigments present is composed of a combination of three distinct types of resins two of which, present in minor proportions, are, respectively, an epoxy ester resin and an aminoplast resin and the third of which, present in predominant proportions, is a particular addition copolymer of a styrenic type monomer and a combination of acrylic type monomers, prepared by aqueous emulsion polymerization and used, as prepared, in the form of an aqueous dispersion or latex.
In the automotive industry it is conventional to spray paint the metal parts of the assembly with a two-coat system consisting of a primer followed by a finish lacquer or enamel of the acrylic type. Primers are used to protect the metal from rust and corrosion, to serve as an adhesion-promoting tie coat between the substrate and the finish lacquer or enamel and as a sandable sealer or surfacer to enhance the final appearance of the painted surface. Compositions for priming automotive assemblies were previously formulated using organic solvents as the volatile carrier vehicle to be removed in the drying or baking cycle, and the equipment has been designed for their use. It is now necessary, because of environmental concerns and other reasons, that the formulation be water-based.
Many water based formulations for metal coating are known to the prior art. Some contain water soluble or dispersible epoxy esters as the film-forming vehicle and others contain relatively low molecular weight addition copolymers of styrenic type monomer with acrylic type monomers prepared by solution polymerization of the monomers in a water miscible organic solvent and modified so as to be reactive or cross-linkable. Aminoplast resins are generally present to assist in the cure. Formulations disclosed are often complex in nature in order to make them adaptable for electrocoating application, as is the case with the formulations disclosed in U.S. Pat. Nos. 3,650,998; 3,652,478 and 3,865,771. While such formulations make use of reactive resinous addition copolymers or styrenic and acrylic monomers, and reactants used for epoxy ester resins, they do not embody the three separately prepared types of resins as above set forth, one of which, the styrenic acrylic copolymer, is present in latex form.
Further, the application of a primer by electrocoating, while quite effective in protecting metal parts against corrosion, is not sufficient to enable elimination of a primer-surfacer under acrylic lacquers or enamels because of the need for better adhesion, and also to provide the "fill" for sanding and smoothing of the primer-surfacer so as to obtain the desired appearance after application of the top coat. There is consequently a need for a water-based primer surfacer composition which can be applied by spraying to automotive assemblies, from which the volatile ingredients are removed in existing equipment, and which meets all the requirements for application under acrylic lacquer or acrylic enamel top coats. No composition has heretofore been found which consistently and adequately serves these purposes.